Conditionally use a thread pool backed sequential executor for DSL rules and events (#3890)

Signed-off-by: Jörg Sautter <joerg.sautter@gmx.net>

bundles/org.openhab.core.automation/src/main/java/org/openhab/core/automation/internal/TriggerHandlerCallbackImpl.java

@@ -13,7 +13,6 @@
 package org.openhab.core.automation.internal;
 
 import java.util.Map;
-import java.util.concurrent.Executors;
 import java.util.concurrent.Future;
 import java.util.concurrent.ScheduledExecutorService;
 
@@ -23,7 +22,7 @@ import org.openhab.core.automation.RuleStatus;
 import org.openhab.core.automation.RuleStatusInfo;
 import org.openhab.core.automation.Trigger;
 import org.openhab.core.automation.handler.TriggerHandlerCallback;
-import org.openhab.core.common.NamedThreadFactory;
+import org.openhab.core.common.ThreadPoolManager;
 
 /**
  * This class is implementation of {@link TriggerHandlerCallback} used by the {@link Trigger}s to notify rule engine
@@ -48,7 +47,7 @@ public class TriggerHandlerCallbackImpl implements TriggerHandlerCallback {
     protected TriggerHandlerCallbackImpl(RuleEngineImpl re, String ruleUID) {
         this.re = re;
         this.ruleUID = ruleUID;
-        executor = Executors.newSingleThreadScheduledExecutor(new NamedThreadFactory("rule-" + ruleUID));
+        this.executor = ThreadPoolManager.getPoolBasedSequentialScheduledExecutorService("rules", "rule-" + ruleUID);
     }
 
     @Override

bundles/org.openhab.core/src/main/java/org/openhab/core/common/PoolBasedSequentialScheduledExecutorService.java

@@ -0,0 +1,487 @@
+/**
+ * Copyright (c) 2010-2024 Contributors to the openHAB project
+ *
+ * See the NOTICE file(s) distributed with this work for additional
+ * information.
+ *
+ * This program and the accompanying materials are made available under the
+ * terms of the Eclipse Public License 2.0 which is available at
+ * http://www.eclipse.org/legal/epl-2.0
+ *
+ * SPDX-License-Identifier: EPL-2.0
+ */
+package org.openhab.core.common;
+
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.IdentityHashMap;
+import java.util.List;
+import java.util.Set;
+import java.util.concurrent.Callable;
+import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.ExecutionException;
+import java.util.concurrent.Executor;
+import java.util.concurrent.Executors;
+import java.util.concurrent.Future;
+import java.util.concurrent.RejectedExecutionException;
+import java.util.concurrent.RunnableFuture;
+import java.util.concurrent.ScheduledExecutorService;
+import java.util.concurrent.ScheduledFuture;
+import java.util.concurrent.ScheduledThreadPoolExecutor;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.TimeoutException;
+import java.util.function.BiFunction;
+import java.util.function.Function;
+
+import org.eclipse.jdt.annotation.NonNullByDefault;
+import org.eclipse.jdt.annotation.Nullable;
+
+/**
+ * A ScheduledExecutorService that will sequentially perform the tasks like a
+ * {@link Executors#newSingleThreadScheduledExecutor} backed by a thread pool.
+ * This is a drop in replacement to a ScheduledExecutorService with one thread to avoid a lot of threads created, idling
+ * most of the time and wasting memory on low-end devices.
+ *
+ * The mechanism to block the ScheduledExecutorService to run tasks concurrently is based on a chain of
+ * {@link CompletableFuture}s.
+ * Each instance has a reference to the last CompletableFuture and will call handleAsync to add a new task.
+ *
+ * @author Jörg Sautter - Initial contribution
+ */
+@NonNullByDefault
+class PoolBasedSequentialScheduledExecutorService implements ScheduledExecutorService {
+
+    private final WorkQueueEntry empty;
+    private final ScheduledThreadPoolExecutor pool;
+    private final List<RunnableFuture<?>> scheduled;
+    private final ScheduledFuture<?> cleaner;
+    private @Nullable WorkQueueEntry tail;
+
+    public PoolBasedSequentialScheduledExecutorService(ScheduledThreadPoolExecutor pool) {
+        if (pool.getMaximumPoolSize() != Integer.MAX_VALUE) {
+            throw new IllegalArgumentException("the pool must scale unlimited to avoid potential dead locks!");
+        }
+
+        this.pool = pool;
+
+        // prepare the WorkQueueEntry we are using when no tasks are pending
+        RunnableCompletableFuture<?> future = new RunnableCompletableFuture<>();
+        future.complete(null);
+        empty = new WorkQueueEntry(null, null, future);
+
+        // tracks scheduled tasks alive
+        this.scheduled = new ArrayList<>();
+
+        tail = empty;
+
+        // clean up to ensure we do not keep references to old tasks
+        cleaner = this.scheduleWithFixedDelay(() -> {
+            synchronized (this) {
+                scheduled.removeIf((sf) -> sf.isCancelled());
+
+                if (tail == null) {
+                    // the service is shutdown
+                    return;
+                }
+
+                WorkQueueEntry entry = tail;
+
+                while (entry.prev != null) {
+                    if (entry.prev.future.isDone()) {
+                        entry.prev = null;
+                        break;
+                    }
+                    entry = entry.prev;
+                }
+
+                if (tail != empty && tail.future.isDone()) {
+                    // replace the tail with empty to ensure we do not prevent GC
+                    tail = empty;
+                }
+            }
+        }, 2, 4, TimeUnit.SECONDS);
+    }
+
+    @Override
+    public ScheduledFuture<?> schedule(@Nullable Runnable command, long delay, @Nullable TimeUnit unit) {
+        return schedule((origin) -> pool.schedule(() -> {
+            // we block the thread here, in worst case new threads are spawned
+            submitToWorkQueue(origin.join(), command).join();
+        }, delay, unit));
+    }
+
+    @Override
+    public <V> ScheduledFuture<V> schedule(@Nullable Callable<V> callable, long delay, @Nullable TimeUnit unit) {
+        return schedule((origin) -> pool.schedule(() -> {
+            // we block the thread here, in worst case new threads are spawned
+            return submitToWorkQueue(origin.join(), callable).join();
+        }, delay, unit));
+    }
+
+    @Override
+    public ScheduledFuture<?> scheduleAtFixedRate(@Nullable Runnable command, long initialDelay, long period,
+            @Nullable TimeUnit unit) {
+        return schedule((origin) -> pool.scheduleAtFixedRate(() -> {
+            CompletableFuture<?> submitted;
+
+            try {
+                // we block the thread here, in worst case new threads are spawned
+                submitted = submitToWorkQueue(origin.join(), command);
+            } catch (RejectedExecutionException ex) {
+                // the pool has been shutdown, scheduled tasks should cancel
+                return;
+            }
+
+            submitted.join();
+        }, initialDelay, period, unit));
+    }
+
+    @Override
+    public ScheduledFuture<?> scheduleWithFixedDelay(@Nullable Runnable command, long initialDelay, long delay,
+            @Nullable TimeUnit unit) {
+        return schedule((origin) -> pool.scheduleWithFixedDelay(() -> {
+            CompletableFuture<?> submitted;
+
+            try {
+                // we block the thread here, in worst case new threads are spawned
+                submitted = submitToWorkQueue(origin.join(), command);
+            } catch (RejectedExecutionException ex) {
+                // the pool has been shutdown, scheduled tasks should cancel
+                return;
+            }
+
+            submitted.join();
+        }, initialDelay, delay, unit));
+    }
+
+    private <V> ScheduledFuture<V> schedule(
+            Function<CompletableFuture<RunnableFuture<?>>, ScheduledFuture<V>> doSchedule) {
+        synchronized (this) {
+            if (tail == null) {
+                throw new RejectedExecutionException("this scheduled executor has been shutdown before");
+            }
+
+            CompletableFuture<RunnableFuture<?>> origin = new CompletableFuture<>();
+            ScheduledFuture<V> future = doSchedule.apply(origin);
+
+            scheduled.add((RunnableFuture<?>) future);
+            origin.complete((RunnableFuture<?>) future);
+
+            return future;
+        }
+    }
+
+    @Override
+    public void shutdown() {
+        synchronized (this) {
+            cleaner.cancel(false);
+            scheduled.removeIf((sf) -> {
+                sf.cancel(false);
+                return true;
+            });
+            tail = null;
+        }
+    }
+
+    @Override
+    @NonNullByDefault({})
+    public List<Runnable> shutdownNow() {
+        synchronized (this) {
+            if (tail == null) {
+                return List.of();
+            }
+
+            // ensures we do not leak the internal cleaner as Runnable
+            cleaner.cancel(false);
+
+            Set<@Nullable Runnable> runnables = Collections.newSetFromMap(new IdentityHashMap<>());
+            WorkQueueEntry entry = tail;
+            scheduled.removeIf((sf) -> {
+                if (sf.cancel(false)) {
+                    runnables.add(sf);
+                }
+                return true;
+            });
+            tail = null;
+
+            while (entry != null) {
+                if (!entry.future.cancel(false)) {
+                    break;
+                }
+
+                if (entry.origin != null) {
+                    // entry has been submitted by a .schedule call
+                    runnables.add(entry.origin);
+                } else {
+                    // entry has been submitted by a .submit call
+                    runnables.add(entry.future);
+                }
+                entry = entry.prev;
+            }
+
+            return List.copyOf(runnables);
+        }
+    }
+
+    @Override
+    public boolean isShutdown() {
+        synchronized (this) {
+            return pool == null;
+        }
+    }
+
+    @Override
+    public boolean isTerminated() {
+        synchronized (this) {
+            return pool == null && tail.future.isDone();
+        }
+    }
+
+    @Override
+    public boolean awaitTermination(long timeout, @Nullable TimeUnit unit) throws InterruptedException {
+        long timeoutAt = System.currentTimeMillis() + unit.toMillis(timeout);
+
+        while (!isTerminated()) {
+            if (System.currentTimeMillis() > timeoutAt) {
+                return false;
+            }
+
+            Thread.onSpinWait();
+        }
+
+        return true;
+    }
+
+    @Override
+    public <T> Future<T> submit(@Nullable Callable<T> task) {
+        return submitToWorkQueue(null, task);
+    }
+
+    private CompletableFuture<?> submitToWorkQueue(RunnableFuture<?> origin, @Nullable Runnable task) {
+        Callable<?> callable = () -> {
+            task.run();
+
+            return null;
+        };
+
+        return submitToWorkQueue(origin, callable);
+    }
+
+    private <T> CompletableFuture<T> submitToWorkQueue(@Nullable RunnableFuture<?> origin, @Nullable Callable<T> task) {
+        BiFunction<? super Object, Throwable, T> action = (result, error) -> {
+            // ignore result & error, they are from the previous task
+            try {
+                return task.call();
+            } catch (RuntimeException ex) {
+                throw ex;
+            } catch (Exception ex) {
+                // a small hack to throw the Exception unchecked
+                throw PoolBasedSequentialScheduledExecutorService.unchecked(ex);
+            }
+        };
+
+        synchronized (this) {
+            if (tail == null) {
+                throw new RejectedExecutionException("this scheduled executor has been shutdown before");
+            }
+
+            RunnableCompletableFuture<T> cf = tail.future.handleAsync(action, pool);
+
+            cf.setCallable(task);
+
+            tail = new WorkQueueEntry(tail, origin, cf);
+
+            return cf;
+        }
+    }
+
+    private static <E extends RuntimeException> E unchecked(Exception ex) throws E {
+        throw (E) ex;
+    }
+
+    @Override
+    public <T> Future<T> submit(@Nullable Runnable task, T result) {
+        return submitToWorkQueue(null, () -> {
+            task.run();
+
+            return result;
+        });
+    }
+
+    @Override
+    public Future<?> submit(@Nullable Runnable task) {
+        return submit(task, (Void) null);
+    }
+
+    @Override
+    @NonNullByDefault({})
+    public <T> List<Future<T>> invokeAll(@Nullable Collection<? extends @Nullable Callable<T>> tasks)
+            throws InterruptedException {
+        List<Future<T>> futures = new ArrayList<>();
+
+        for (Callable<T> task : tasks) {
+            futures.add(submit(task));
+        }
+
+        // wait for all futures to complete
+        for (Future<T> future : futures) {
+            try {
+                future.get();
+            } catch (ExecutionException e) {
+                // ignore, we are just waiting here for the futures to complete
+            }
+        }
+
+        return futures;
+    }
+
+    @Override
+    @NonNullByDefault({})
+    public <T> List<Future<T>> invokeAll(@Nullable Collection<? extends @Nullable Callable<T>> tasks, long timeout,
+            TimeUnit unit) throws InterruptedException {
+        List<Future<T>> futures = new ArrayList<>();
+
+        for (Callable<T> task : tasks) {
+            futures.add(submitToWorkQueue(null, task).orTimeout(timeout, unit));
+        }
+
+        // wait for all futures to complete
+        for (Future<T> future : futures) {
+            try {
+                future.get();
+            } catch (ExecutionException e) {
+                // ignore, we are just waiting here for the futures to complete
+            }
+        }
+
+        return futures;
+    }
+
+    @Override
+    public <T> T invokeAny(@Nullable Collection<? extends @Nullable Callable<T>> tasks)
+            throws InterruptedException, ExecutionException {
+        try {
+            return invokeAny(tasks, Long.MAX_VALUE);
+        } catch (TimeoutException ex) {
+            throw new AssertionError(ex);
+        }
+    }
+
+    @Override
+    public <T> T invokeAny(@Nullable Collection<? extends @Nullable Callable<T>> tasks, long timeout,
+            @Nullable TimeUnit unit) throws InterruptedException, ExecutionException, TimeoutException {
+        long timeoutAt = System.currentTimeMillis() + unit.toMillis(timeout);
+
+        return invokeAny(tasks, timeoutAt);
+    }
+
+    private <T> T invokeAny(@Nullable Collection<? extends @Nullable Callable<T>> tasks, long timeoutAt)
+            throws InterruptedException, ExecutionException, TimeoutException {
+        List<CompletableFuture<T>> futures = new ArrayList<>();
+
+        for (Callable<T> task : tasks) {
+            futures.add(submitToWorkQueue(null, task));
+        }
+
+        // wait for any future to complete
+        while (timeoutAt >= System.currentTimeMillis()) {
+            boolean allDone = true;
+
+            for (CompletableFuture<T> future : futures) {
+                if (future.isDone()) {
+                    if (!future.isCompletedExceptionally()) {
+                        // stop the others
+                        for (CompletableFuture<T> tooLate : futures) {
+                            if (tooLate != future) {
+                                tooLate.cancel(true);
+                            }
+                        }
+
+                        return future.join();
+                    }
+                } else {
+                    allDone = false;
+                }
+            }
+
+            if (allDone) {
+                ExecutionException exe = new ExecutionException("all tasks failed", null);
+
+                for (CompletableFuture<T> future : futures) {
+                    try {
+                        future.get();
+                        throw new AssertionError("all tasks should be failed");
+                    } catch (ExecutionException ex) {
+                        exe.addSuppressed(ex);
+                    }
+                }
+
+                throw exe;
+            }
+
+            Thread.onSpinWait();
+        }
+
+        for (CompletableFuture<T> tooLate : futures) {
+            tooLate.cancel(true);
+        }
+
+        throw new TimeoutException("none of the tasks did complete in time");
+    }
+
+    @Override
+    public void execute(Runnable command) {
+        submit(command);
+    }
+
+    static class WorkQueueEntry {
+        private @Nullable WorkQueueEntry prev;
+        private @Nullable RunnableFuture<?> origin;
+        private final RunnableCompletableFuture<?> future;
+
+        public WorkQueueEntry(@Nullable WorkQueueEntry prev, @Nullable RunnableFuture<?> origin,
+                RunnableCompletableFuture<?> future) {
+            this.prev = prev;
+            this.origin = origin;
+            this.future = future;
+        }
+    }
+
+    static class RunnableCompletableFuture<V> extends CompletableFuture<V> implements RunnableFuture<V> {
+        private @Nullable Callable<V> callable;
+
+        public RunnableCompletableFuture() {
+            callable = null;
+        }
+
+        public void setCallable(@Nullable Callable<V> callable) {
+            this.callable = callable;
+        }
+
+        @Override
+        public <U> RunnableCompletableFuture<U> newIncompleteFuture() {
+            return new RunnableCompletableFuture<>();
+        }
+
+        @Override
+        public <U> RunnableCompletableFuture<U> handleAsync(BiFunction<? super V, Throwable, ? extends U> fn,
+                Executor executor) {
+            return (RunnableCompletableFuture<U>) super.handleAsync(fn, executor);
+        }
+
+        @Override
+        public void run() {
+            if (this.isDone()) {
+                // a FutureTask does also return here without exception
+                return;
+            }
+
+            try {
+                this.complete(callable.call());
+            } catch (Error | Exception t) {
+                this.completeExceptionally(t);
+            }
+        }
+    }
+}

bundles/org.openhab.core/src/main/java/org/openhab/core/common/ThreadPoolManager.java

@@ -22,6 +22,7 @@ import java.util.concurrent.Callable;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.ExecutionException;
 import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
 import java.util.concurrent.Future;
 import java.util.concurrent.ScheduledExecutorService;
 import java.util.concurrent.ScheduledFuture;
@@ -115,6 +116,27 @@ public class ThreadPoolManager {
         }
     }
 
+    /**
+     * Returns an instance of a scheduled service, which will sequentially execute submitted tasks. If a task is
+     * currently running the task is queued until the previous one is completed, this also applies for scheduled tasks.
+     * The service might execute submitted task might in different threads, but still one after the other.
+     * If it is the first request for the given pool name and a pool is used, the instance is newly created.
+     *
+     * @param poolName a short name used to identify the pool, if a thread pool is used e.g. "bluetooth-discovery"
+     * @param threadName a short name used to identify the thread if no thread pool is used, e.g. "bluetooth"
+     * @return an instance to use
+     */
+    public static ScheduledExecutorService getPoolBasedSequentialScheduledExecutorService(String poolName,
+            String threadName) {
+        if (configs.getOrDefault(poolName, 0) > 0) {
+            ScheduledThreadPoolExecutor pool = getScheduledPoolUnwrapped(poolName);
+
+            return new PoolBasedSequentialScheduledExecutorService((ScheduledThreadPoolExecutor) pool);
+        } else {
+            return Executors.newSingleThreadScheduledExecutor(new NamedThreadFactory(threadName));
+        }
+    }
+
     /**
      * Returns an instance of a scheduled thread pool service. If it is the first request for the given pool name, the
      * instance is newly created.

bundles/org.openhab.core/src/main/java/org/openhab/core/internal/events/EventHandler.java

@@ -29,6 +29,7 @@ import java.util.concurrent.atomic.AtomicInteger;
 import org.eclipse.jdt.annotation.NonNullByDefault;
 import org.eclipse.jdt.annotation.Nullable;
 import org.openhab.core.common.NamedThreadFactory;
+import org.openhab.core.common.ThreadPoolManager;
 import org.openhab.core.events.Event;
 import org.openhab.core.events.EventFactory;
 import org.openhab.core.events.EventFilter;
@@ -69,9 +70,8 @@ public class EventHandler implements AutoCloseable {
     }
 
     private synchronized ExecutorRecord createExecutorRecord(Class<? extends EventSubscriber> subscriber) {
-        return new ExecutorRecord(
-                Executors.newSingleThreadExecutor(new NamedThreadFactory("eventexecutor-" + executors.size())),
-                new AtomicInteger());
+        return new ExecutorRecord(ThreadPoolManager.getPoolBasedSequentialScheduledExecutorService("events",
+                "eventexecutor-" + executors.size()), new AtomicInteger());
     }
 
     @Override

bundles/org.openhab.core/src/test/java/org/openhab/core/common/ThreadPoolManagerTest.java

@@ -17,13 +17,20 @@ import static org.hamcrest.MatcherAssert.assertThat;
 import static org.junit.jupiter.api.Assertions.*;
 
 import java.util.Map;
+import java.util.concurrent.Callable;
+import java.util.concurrent.CompletableFuture;
 import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.RunnableFuture;
 import java.util.concurrent.ScheduledExecutorService;
 import java.util.concurrent.ThreadPoolExecutor;
 import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicBoolean;
 
 import org.eclipse.jdt.annotation.NonNullByDefault;
+import org.junit.jupiter.api.AfterAll;
+import org.junit.jupiter.api.BeforeAll;
 import org.junit.jupiter.api.Test;
 
 /**
@@ -35,6 +42,138 @@ import org.junit.jupiter.api.Test;
 @NonNullByDefault
 public class ThreadPoolManagerTest {
 
+    @BeforeAll
+    public static void enableSequentialScheduledExecutorService() {
+        ThreadPoolManager manager = new ThreadPoolManager();
+        manager.activate(Map.of("unit-test", "10"));
+    }
+
+    @Test
+    public void testSequentialExecutorServiceAssumptions() {
+        Callable<Boolean> callable = () -> true;
+        Runnable runnable = () -> {
+        };
+
+        ScheduledExecutorService service = Executors.newScheduledThreadPool(1);
+
+        assertTrue(service.submit(runnable) instanceof RunnableFuture);
+        assertTrue(service.submit(callable) instanceof RunnableFuture);
+
+        assertTrue(service.schedule(runnable, 1, TimeUnit.SECONDS) instanceof RunnableFuture);
+        assertTrue(service.schedule(callable, 1, TimeUnit.SECONDS) instanceof RunnableFuture);
+
+        var fixedRate = service.scheduleAtFixedRate(runnable, 1, 1, TimeUnit.SECONDS);
+
+        try {
+            assertTrue(fixedRate instanceof RunnableFuture);
+        } finally {
+            fixedRate.cancel(false);
+        }
+
+        var fixedDelay = service.scheduleWithFixedDelay(runnable, 1, 1, TimeUnit.SECONDS);
+
+        try {
+            assertTrue(fixedDelay instanceof RunnableFuture);
+        } finally {
+            fixedDelay.cancel(false);
+        }
+
+        service.shutdownNow();
+    }
+
+    @Test
+    public void testExecutionIsSequentialInSequentialExecutorService() {
+        ScheduledExecutorService service = ThreadPoolManager.getPoolBasedSequentialScheduledExecutorService("unit-test",
+                "thread-1");
+
+        AtomicBoolean done = new AtomicBoolean(false);
+
+        service.submit(() -> {
+            Thread.sleep(100);
+
+            done.set(true);
+
+            return null;
+        });
+
+        assertTrue(((CompletableFuture<Boolean>) service.submit(() -> done.get())).join());
+    }
+
+    @Test
+    public void testCancelDoesNotStopProcessingInSequentialExecutorService() throws InterruptedException {
+        CountDownLatch latch = new CountDownLatch(1);
+        ScheduledExecutorService service = ThreadPoolManager.getPoolBasedSequentialScheduledExecutorService("unit-test",
+                "thread-2");
+
+        service.submit(() -> {
+            Thread.sleep(100);
+
+            return null;
+        });
+
+        service.submit(() -> null).cancel(false);
+
+        service.submit(() -> {
+            latch.countDown();
+        });
+
+        assertTrue(latch.await(1, TimeUnit.SECONDS));
+    }
+
+    @Test
+    public void testInstancesDoNotBlockEachOtherInSequentialExecutorService() throws InterruptedException {
+        ScheduledExecutorService serviceA = ThreadPoolManager
+                .getPoolBasedSequentialScheduledExecutorService("unit-test", "thread-3");
+        ScheduledExecutorService serviceB = ThreadPoolManager
+                .getPoolBasedSequentialScheduledExecutorService("unit-test", "thread-4");
+
+        serviceA.submit(() -> {
+            Thread.sleep(100);
+
+            return null;
+        });
+
+        CountDownLatch latch = new CountDownLatch(1);
+
+        serviceB.submit(() -> {
+            latch.countDown();
+        });
+
+        assertTrue(latch.await(800, TimeUnit.MILLISECONDS));
+    }
+
+    @Test
+    public void testSchedulingWorksInSequentialExecutorService() throws InterruptedException {
+        ScheduledExecutorService service = ThreadPoolManager.getPoolBasedSequentialScheduledExecutorService("unit-test",
+                "thread-5");
+        CountDownLatch latch = new CountDownLatch(1);
+
+        service.schedule(() -> {
+            latch.countDown();
+        }, 200, TimeUnit.MILLISECONDS);
+
+        assertTrue(latch.await(1, TimeUnit.SECONDS));
+    }
+
+    @Test
+    public void testSchedulingGetsBlockedByRegularTaskInSequentialExecutorService() throws InterruptedException {
+        ScheduledExecutorService service = ThreadPoolManager.getPoolBasedSequentialScheduledExecutorService("unit-test",
+                "thread-6");
+        CountDownLatch latch = new CountDownLatch(1);
+
+        service.submit(() -> {
+            Thread.sleep(200);
+
+            return null;
+        });
+
+        service.schedule(() -> {
+            latch.countDown();
+        }, 20, TimeUnit.MILLISECONDS);
+
+        assertFalse(latch.await(100, TimeUnit.MILLISECONDS));
+    }
+
     @Test
     public void testGetScheduledPool() {
         ThreadPoolExecutor result = ThreadPoolManager.getScheduledPoolUnwrapped("test1");
@@ -136,4 +275,10 @@ public class ThreadPoolManagerTest {
         assertTrue(cdl.await(5, TimeUnit.SECONDS), "Checking if thread pool " + poolName + " works");
         assertFalse(threadPool.isShutdown(), "Checking if thread pool is not shut down");
     }
+
+    @AfterAll
+    public static void disableSequentialScheduledExecutorService() {
+        ThreadPoolManager manager = new ThreadPoolManager();
+        manager.activate(Map.of("unit-test", "0"));
+    }
 }

tools/static-code-analysis/pmd/suppressions.properties

@@ -15,3 +15,4 @@ org.openhab.core.automation.internal.RuleEngineImpl=AvoidCatchingThrowable
 org.openhab.core.automation.internal.RuleRegistryImpl=CompareObjectsWithEquals
 org.openhab.core.automation.internal.provider.AutomationResourceBundlesEventQueue=AvoidCatchingThrowable
 org.openhab.core.io.console.karaf.internal.InstallServiceCommand=SystemPrintln
+org.openhab.core.common.PoolBasedSequentialScheduledExecutorService=CompareObjectsWithEquals